Ecological zonation of salt marsh macrophytes is strongly influenced by hydrologic factors, but these factors are poorly understood. We examined groundwater flow patterns through surficial sediments in two saltmarshes in the southeastern United States to quantify hydrologic differences between distinct ecological zones. Both sites included tall- or medium-form Spartina alterniflora near the creek bank; short-form Spartina alterniflora in the mid-marsh; salt flats and Salicornia virginica in the high marsh; and Juncus roemarianus in brackish-to-fresh areas adjacent to uplands. Both sites had relatively small, sandy uplands and similar stratigraphy consisting of marsh muds overlying a deeper sand layer. We found significant hydrologic differences between the four ecological zones. In the zones colonized by S. alterniflora, the vertical flow direction oscillated with semi-diurnal tides. Net flow (14-day average) through the tall S. alterniflora zones was downward, whereas the short S. alterniflora zones included significant periods of net upward groundwater flow. An examination of tidal efficiency at these sites suggested that the net flow patterns rather than tidal damping controlled the width of the tall S. alterniflora zone. In contrast to the S. alterniflora zones, hypersaline zones populated by S. virginica were characterized by sustained periods (days) of continuous upward flow of saline water during neap tides. The fresher zone populated by J. roemarianus showed physical flow patterns that were similar to the hypersaline zones, but the upwelling porewaters were fresh rather than saline. These flow patterns were influenced by the hydrogeologic framework of the marshes, particularly differences in hydraulic head between the upland water table and the tidal creeks. We observed increases in hydraulic head of approximately 40 cm from the creek to the upland in the sand layers below both marshes, which is consistent with previous observations that sandy aquifers below fine-grained marsh soils act as conduits for flow from uplands to tidal creeks. This hydrologic framework supports relatively good drainage near the creek, increased waterlogging in the mid-marsh, and the development of hypersalinity adjacent to the freshwater upland. These hydrologic differences in turn support distinct ecological zones.

[1] Storms can cause significant groundwater flow in coastal settings, but prior studies of the effects of storms on groundwater flow and transport have largely focused on very large storms and used salinity as a tracer. We have little information about the effects of smaller storms on coastal flow and how storm-induced variability affects key tidal wetlands like salt marshes, which may remain saline throughout a storm. Here we show that even the distant passage of a moderate storm can strongly increase groundwater flow and transport in salt marsh ecosystems and adjacent barrier islands. Groundwater monitoring and radium isotope tracer analyses revealed significant influx of saline creek water into the confined aquifer below the marsh platform, driven by storm surge. This pulse of fluids reached depths exceeding 5 m, and surge-enhanced tides propagated through the aquifer to affect flow in the upland >100 m from the creek bank. Groundwater discharge from the marsh varied significantly prior to the storm, doubling during inundating tides compared to a period of noninundating neap tides. Storm surge then caused groundwater discharge to decline $50% compared to similar inundating tides. Ra-and nutrient-poor creek water that entered the confined aquifer below the marsh was quickly enriched in nutrients and carbon, even on 12 h tidal cycles, so that nutrient discharge was likely proportional to groundwater discharge. Storm-related flow could also drive significant contaminant discharge from developed coastlines. The enhanced transport and variability observed here likely affected hundreds of kilometers of the coastline impacted by the storm.

Changes in stream temperatures in response to restoration of groundwater discharge and solar heating in a culverted, urban stream A B S T R A C TBoone Creek is a mountainous headwater stream that lies within an urbanized environment in north-western North Carolina. The primary source of thermal pollution in Boone Creek is the urban infrastruc-ture, which affects stream temperatures through (1) heated runoff, which creates temperature surges and (2) the elimination of groundwater-surface water interactions. In this study, we use a Monte Carlo ther-mal mixing model to predict the thermal impact of removing a 700-m-long culvert. Our thermal mixing model balances stream discharge and temperatures with surface-heat exchange parameters and restored baseflow. We calculate the daily-average groundwater discharge velocity at 15 locations in the stream using signal decay in streambed temperatures, and utilize a Monte Carlo implementation of the hetero-geneous baseflow field in the thermal mixing model. We also calculate surface-heat exchange per unit area for conditions upstream and downstream of the existing culvert and utilize those values in the ther-mal mixing model. Our modeled temperatures suggest a decrease in summer stream temperatures down-stream of the culvert that average 1.35 ° C and 1.17 ° C for upstream and downstream surface-heat exchange conditions, respectively. The results of our study have implications for the balance between baseflow and the urban infrastructure in any high-gradient urban stream that experiences similar ther-mal effects.

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